[lldb-dev] Support of a lookup of variables values with PDB

Aleksandr Urakov via lldb-dev lldb-dev at lists.llvm.org
Thu Jun 28 00:32:39 PDT 2018

Thank you for explanations!

I'll create the converter to DWARF expressions.


On Wed, Jun 27, 2018 at 5:38 PM Greg Clayton <clayborg at gmail.com> wrote:

> On Jun 27, 2018, at 5:25 AM, Aleksandr Urakov via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> Hello!
> We want to add to LLDB a support of a lookup of variables values with PDB.
> Now SymbolFilePDB::ParseVariableForPDBData function uses an empty location
> for variables, so e.g. `fr v` prints values as '<empty constant data>'. Symbol
> location information is available in a PDB (through
> PDBSymbolData::getLocationType and so on), but not in the format of DWARF
> expression. Do I understand correctly, that it is necessary to write some
> converter of a PDB symbol location to a DWARF expression bytecode? Is this
> the preferable way of solving the issue? What are pitfalls there? Please,
> share your thoughts on this.
> DWARF expressions are pretty powerful and provide all the opcode need to
> describe variable locations. You will need to covert the PDB location to
> the DWARF expression opcodes.
> Some examples:
> variable is at SP + 4 = DW_OP_bregXXX(4)
> DW_OP_bregXXX where XXX is the register number for your stack pointer.
> This opcode takes a single operand that is the offset from the register in
> question. The variable value is expected to be at the load address in
> memory.
> variable is in register 12 = DW_OP_reg12
> There are 32 DW_OP_regXXX opcodes DW_OP_reg0 - DW_OP_reg31. If your
> register value is higher than 32, then use DW_OP_regx which takes a ULEB128
> parameter which is the register number:
> variable is in register 46 = DW_OP_regx(46)
> Global variables:
> variable is at file address 0x12340000 = DW_OP_addr(0x12340000)
> The address here is a file address, or the file VM address of the global
> as it is known in the object file. This address will be slid and covered to
> a load address if needed at runtime.
> IF you have any other questions on converting your locations, let me know.
> But the above examples should give you a good start.
> Greg
> Regards,
> Aleksandr
> --
> Aleksandr Urakov
> Software Developer
> JetBrains
> http://www.jetbrains.com
> The Drive to Develop
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Aleksandr Urakov
Software Developer
The Drive to Develop
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